The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a process for fabricating a semiconductor device having a redundancy circuit.
In recent years, a redundancy technique has been employed in the manufacturing process of semiconductor devices. The reason for this is that the redundancy technique allows fail bits associated with random-defects, which are found in the semiconductor device fabricated, to be replaced by spare elements, thereby improving a manufacturing yield. One of the known methods for replacing the defective bits with the correct spare elements is that a polysilicon (polycrystalline silicon) film resistor made of polysilicon or the like connecting a correct element to an integrated circuit is fused by conduction of electrical current or irradiation by a laser. For fusing the polysilicon film resistor through the conduction of electricity, a circuit for suppling current is required and a large capacitive transistor must be provided for feeding a large current. During the process of fusing the film resistor by the conduction of electricity or laser irradiation the resultant melted material is scattered on its peripheral circuits. More adversely, at the time of the fusing, the peripheral circuits and/or layers under the film resistor may be destroyed. Further, microdefects may be formed in the structure near the film resistor due to the fusing of the film resistor, thus resulting in degradation of the reliability of the semiconductor devices manufactured.
In 1981 IEEE International Solid-State Circuits Conference, a research worker in Hitachi Central Research Laboratory disclosed a technique to transform a polysilicon film into a conductive material by diffusing an impurity into the polysilicon film on both sides by application of lower pulses. For further details of this technique, reference is made to ISSCC Digest of Technical Papers P14, Feb. 1981.
In the redundancy technique, for column decoders, spare elements are programmed by applying laser pulses to an intrinsic polysilicon film having an N.sup.+ diffusion on either side, and by electrically connecting the two N.sup.+ diffusions. By this process, the intrinsic polysilicon film having a resistivity of more than 10.sup.9 ohms is changed to a conductivity film with a resistivity of less than 10.sup.3 ohms.